`rep stosb`/`rep movsb` are exactly what real bootloaders and even modern glibc's memcpy/memset fast paths use under the hood for bulk memory operations, because these microcoded instructions have historically outperformed hand-rolled copy loops. Forgetting CLD before a string operation is a real, well-known bug: DF's state is undefined after boot, so a stray STD left set by BIOS code can silently reverse every subsequent MOVSB/STOSB in your program.